The invention relates to additive color systems of the type operative for independently attenuating three component beams of light and then combining the component beams to form a composite beam of white light having a corrected color balance.
Such systems are typically used when duplicating color transparencies by printing. Conventionally, each transparency is analyzed and a determination is made of what corrections are required for the red, green and blue components of the white exposure light used for the actual printing. However, such systems are also used in a wide variety of other applications, wherever it is necessary to be able to independently select the color components of a beam of light.
Conventionally, the attenuation of the component light beams is performed using adjustable diaphragms, electromechanical light valves or sets of neutral filters of different density.
However, particularly when the additive color system is used to supply color-corrected white light to a high-speed printer, the time required to change the density of mechanical or electromechanical attenuators will often be so great as to prevent the fullest utilization of the high-speed capability of the printer.
As a result, I have considered the possibility of using for the controllable attenuators recently developed controllable-density electrooptical filter units. Examples of such filter units include ferroelectric-ceramic, liquid crystal and suspended-dipole filter units. Filter units of the first two types include a pair of spaced polarizers, whose polarization directions are usually perpendicular to each other, with an intermediate plate or layer of ferroelectric ceramic or liquid crystal material provided on its surface with transparent control electrodes. Suspended-dipole filter units, in contrast, do not require the use of polarizers, and are comprised of a layer of liquid in which are suspended submicron needle-like crystal dipoles, sandwiched between transparent plates the inner sides of which are coated with transparent control electrodes.
In principle, these electrooptical filter units are capable of very high speed operation, and are therefore well suited to high-speed printing applications, and the like, where the transmission level of the filter unit is to be changed repeatedly at short time intervals. In practice, however, I have found that various properties of these electrooptical (electrical-field operated) optical filter units present very serious problems of repeatability. In some instances, these repeatability difficulties are so considerable as to virtually preclude sufficiently precise control of the transmission levels of the filter units.
For example, in the case of filter units provided with a layer of ferroelectric ceramic material of controllable birefringence, I have found that hysteresis effects are so considerable that there is often no one-to-one correspondence between the actual transmission level of the filter unit, on the one hand, and the value of the control voltage applied to the control electrodes of the ferroelectric ceramic layer, on the other hand. Thus, if a first control voltage is applied to establish a first degree of birefringence, and if then a second control voltage is applied to establish a second degree of birefringence, the degree of birefringence achieved in the second case depends not only upon the value of the second control voltage, but also upon the value of the first control voltage, and upon whether the second control voltage was higher than the first, or vice versa. Without a one-to-one correspondence between the control voltage value and the resultant transmission level, precise control of the density of the filter unit cannot be readily achieved.
In addition to hysteresis, the electrooptical material of the filter unit may exhibit very considerable non-linearity in the relationship between the value of the applied control voltage and the resultant degree of birefringence, or more generally, between the control voltage value and the resultant transmission level of the filter unit. This can be the case irrespective of whether the filter unit also exhibits non-negligible hysteresis errors.
With a non-linear control-voltage/transmission-level relationship, it is in general necessary to expressly take the non-linearity into account when selecting the value for the control voltage. The provision of transmission-level selectors designed to exhibit corresponding non-linearity in the relationship between the selected transmission level and the value of the control voltage produced by the selector, is in itself troublesome. In addition, if the non-linearity in question varies from one filter unit to the next, then the selector must be specifically modified for the filter unit with which it is to cooperate.